Mobile handsets today are often required to address multiple RF standards, such as 2G (second generation), 3G, 4G, GPS (global positioning system), WiFi (wireless fidelity), Bluetooth, etc. In order to keep manufacturing and test costs as low as possible, the number of discrete devices used to implement an RF receiver for a given standard has shrunk from several hundreds to just a few tens. There is also a move towards fully-integrated solutions.
A classical solution is to use a super-heterodyne architecture to down-convert the RF signal to a given intermediate frequency (IF). The RF channel selection in architectures of this type could be performed using one of two different approaches.
One approach would be to use an RF clock synthesizer to down-convert the signal to the given IF, and use an external off-chip VCO (voltage controlled oscillator) component, so as to provide a local oscillator for mixing the signal and performing down conversion. In addition, it is typical to use a quartz filter, centered at the IF, to filter out any interferers. The main limitation of this solution is that a high phase noise requirement prevents the integration of VCO itself. Furthermore, the lack of re-configurability of the off-chip component prevents several standards having different channel bandwidths from being addressed with a single component.
An alternative approach would be to use a fixed frequency synthesizer to perform the down-conversion, and performing the filtering using a versatile filter. However, reconfigurability of the central frequency is practically impossible to achieve in such a case, and the constraint in terms of bandwidth is extremely high.
The publication by Pui-In Mak et al. entitled “Two-step Channel Selection—A Novel Technique for Reconfigurable Multistandard Transceiver Front-Ends”, IEEE Transactions on Circuits and Systems-I: Regular papers, Vol. 52, No. 7, July 2005, proposes a partition of the channel selection process between RF and IF analog front-ends, such that only a coarse selection is necessary at the RF, and a fine selection is completed at the IF.
However, while the solution described by Mak et al. provides a good trade-off between the two previously described solutions, it has drawbacks in terms of the accuracy of the frequency selection, and the quality factor that can be achieved.
There is thus a need for an alternative receiver architecture providing channel selection with improved precision and a higher quality factor with respect to existing solutions, and for a solution permitting reconfigurability for multistandard operation.